There are two ways one of your campaigns can get forwarded.
The first is simply that an excited recipient thinks your email is useful and they forward it directly onward. The original email along with the original, unmodified trackable links and embedded 'beacon' image (used to track HTML email opens) is forwarded. When the *new* recipient (person email was forwarded to) opens the email or clicks a link, this will register as an open or click for the original recipient. When reviewing statistics, what you'll see is repeated opens/clicks registered for the same recipient.
Here's an example from Pinpointe's email open reporting. Maybe this person was click happy. Maybe their cat walked across their keyboard a few times and stepped on the wrong keys. But most likely, they forwarded the email to a few people (might have been because we embedded a darn funny Youtube video):
The second way an email can get forwarded is with the Forward-to-a-Friend form, which is trackable.
Most ESP's include a 'Forward-to-a-Friend' form or link within their email. Now if this link is used for forwarding, then the happy forwarded can enter the email addresses of multiple recipients in an online form (automatically generated and hosted by the ESP), and the individual forwarded stats can be tracked
If an email is simply forwarded, and the recipient opens and/or clicks on links, the open and click will be registered There is presently not a method to track email opens for text emails; however Pinpointe can track link clicks even if the content is 100% text.